In RF communications, interference between differing carrier frequencies is undesirable, even when the information is coded in FM or more sophisticated systems such as CDMA, which are generally more immune to interference compared to classical AM systems. For instance, when using CDMA, different carrier frequencies are often spaced closely together, which can result in leakage of signals between carriers, which can corrupt the coding contained in the carrier. The carriers should be amplified before being transmitted at the power level dictated by the communication channel. In that process, interference can arise due to the non-linearity of RF power amplifiers. Nonlinearity can result in generation of harmonics of the carrier frequencies and can cause damaging intermodulation between carriers. This is demonstrated by the amplifier output voltage expressed by equation (1), showing it is composed by an amplification of input voltage, a1, plus additional (undesirable) components, a2 and a3. Of these additional undesirable components, the most problematic is a3, which creates the so-called third order intermodulation component.Vout=a1Vin+a2Vin2+a3Vin3+  (1)
A first value Pout represents a desired output signal for an amplifier, proportional to the input signal by an amplification constant a1, as described in equation (1) above. A second value IMD3 represents an undesired signal contributed to an output of an amplifier proportional to the input signal by an third-order intermodulation component a3. A linearity parameter P1 dB indicates an output power level when the gain of the amplifier is reduced by 1 dB from Pout. Another linearity parameter OIP3 (Output Intercept Point of 3rd order) indicates a extrapolated intersection of the linear contribution of Pout and the undesired contribution from IMD3.
In communication systems such as WCDMA, WiMax, or LTE systems, it is desirable to have the ratio IMD3/Pout be as low as possible, which is directly related to the ratio a3/a1, so that the undesired signals are low enough to not affect recovery of information at the receiver. One known amplification topology used in RF communication is the cascode amplifier, in which two amplifiers are coupled in series, with the effect of having the same current circulating in each device. While these known amplification circuits generally achieve the effect of relatively high input-to-output isolation and relatively high gain, they have drawbacks, among which are inferior linearity when compared with conventional single-ended amplifiers operating at the same power supply voltage, such as about +5 volts.
Some publications which might have bearing on the known art include:    “High Linearity Cascode Low Noise Amplifier”, U.S. Pat. No. 6,392,492 Bi (May 21, 2002), Xiaojuen Yuan.    ANALYSIS AND DESIGN OF ANALOG INTEGRATED CIRCUITS (John Wiley & Sons, 2001), Paul R. Gray, Paul J. Hurst, Stephen H. Lewis, Robert G. Meyer.